Heretofore, as an applied product of an ultrasonic motor, a spherical ultrasonic motor (spherical actuator) is available, which is constituted of a substantially spherical rotor and a plurality of ring-like stators contacting surfaces of the rotor and rotatably supporting the rotor. Each stator is constituted by attaching a piezoelectric element constituted of a piezoelectric ceramic to a ring-like metal. The piezoelectric element has an electrode pattern including an A-phase and a B-phase, and the polarization polarity of the electrode pattern is made so as to alternately differ in the circumferential direction of the ring-like stator by a polarization treatment. If two-phase high-frequency voltages which are different in phase by 90 degrees are applied to the A-phase and B-phase of the electrode pattern of the piezoelectric element, respectively, the stator is put in a resonant condition at a predetermined frequency. As a result, travelling waves due to flexural vibrations are generated in the stator, and the rotor pressurized by the stator is rotated by friction forces. Thus, in a spherical ultrasonic motor, surface wave vibrations generated by plural stators are conveyed to a rotor, and the rotor is rotated in multiple directions by the surface wave vibrations.
By the way, speed control of an ultrasonic motor is carried out basically by two methods, that is, a method controlling phases of voltages applied to piezoelectric elements constituting stators, and a method controlling frequencies of the voltages applied to the piezoelectric elements.
The phase refers to a phase difference of a two-phase voltage which is applied to a stator (piezoelectric element) which generates vibrations. Vibrations on the surface of the stator become standing waves (the rotor does not rotate) when the phase difference of a two-phase voltage which is applied to a stator is 0°, and when the phase difference is 90°, the vibrations on the surface of the stator become traveling waves (the amplitude is constant and the rotor swiftly rotates). Using this phenomenon, it is practiced to control the rotation speed of a rotor by adjusting phase differences of two-phase voltages which are applied to stators.
On the other hand, in the method controlling the rotation speed of a rotor by adjusting frequencies of two-phase voltages applied to stators, when frequencies of applied voltages are resonant frequencies of the stators, the rotor rotates most often, and as the frequencies of the applied voltages are shifted from the resonant frequencies, the rotation speed of the rotor decreases, and the rotor stops in due course of time. Using this, it is practiced to control the rotation speed of a rotor by controlling frequencies of two-phase voltages applied to stators.
By the way, the spherical ultrasonic motor is configured such that a substantially spherical rotor is held, for example, by three disk-like stators, and the rotor is rotated around a resultant vector of angular velocity vectors of respective stators. Heretofore, by adjusting phases of two-phase voltages applied to the three stators, the rotation axis direction and the rotation speed of the rotor have been controlled. This is because that in the case of a spherical ultrasonic motor, when controlling stators based on frequencies of two-phase voltages applied to the stators, if the frequencies are small, amplitudes of surface wave vibrations become small and interference between the stators is strong, so that generation of a rotation axis of the rotor has been difficult, and also the rotation speed of the rotor has been unstable.
As an example of controlling a spherical actuator based on phases of two-phase voltages applied to stators, for example, a control technology is disclosed, in which rotation of a rotor is controlled using phase differences of two-phase high-frequency voltages which are applied to respective stators of the spherical actuator (for example, see Patent Document 1).
Patent Document 1: Japanese Unexamined Patent Application Publication No. 11-84526